Here you can see a page from Thomas Huxley's Evidence As To Man's Place In Nature, where Darwin has scribbled "sexual selection" in the margin and underlined passages he found interesting.

Every comment he made has also been transcribed, so you don't have to decipher his scribbles. According to a release from the library:

The series of transcriptions accompanying each page allows everyone to see which passages Darwin found relevant to his work, stimulated his thinking, or just annoyed him as he read the work of others.

For example, his friend Charles Lyell wrote in his famous Principles of Geology that there were definite limits to the variation of species. Darwin wrote alongside this: "If this were true adios theory."